Reply I sent on a similar thread

On the base (underside) of the tap there are usually three threaded
holes, two are either 10mm or 12mm threaded these are for the feed
pipes and one is a smaller (8mm??) threaded hole. Into this goes a
short length of threaded bar, the tap then goes through the sink
hole, and a 'half moon' shaped rubber washer and a 'half moon shaped
steel washer go onto this threaded bar, rubber one against the
underside of the sink. The whole thing is then tightened with a 11mm
diameter (usually) elongated nut. All these bits are supplied with
the tap. The awkward part is tightening this up while contorted into
a kitchen cupboard. I usually, and you can too if confident enough,
cut the threaded bar down and then you can use a socket set on the
nut, but make sure you don't cut the bar too short or you'll not get
the nut on!

I hope this helps you, I have tried to find an online picture of the
bits but haven't come up with anything, sorry.

John

Thanks John , my tap is fitted correctly reading your info.

The problem must be the stainless steel being too thin, not
rigid enough to hold the monobloc steady.
I think I will try to use the threaded bar to attach a tie rod
and fix to back of unit or wall ... might help.

If it wasn't for the expense of a new sink I would prefer to
revert to normal taps, monoblocs are a flood waiting to happen.